In recent years, there are rapidly increasing demands for smaller and more sophisticated electronic components, for example, power devices and light-emitting diodes (LEDs). Power devices as semiconductor devices capable of efficiently performing electric power conversion with reduced power loss are becoming more popular in the fields of electric vehicles, hybrid vehicles, quick chargers, and the like. Further, they are likely to be more demanded in the field of new energy such as solar power systems and mega solar systems.
Meanwhile, LED devices, which have advantages such as a longer service life, a smaller size, and lower power consumption as compared with incandescent bulbs, are rapidly becoming more popular in various fields such as illumination, mobile phones, liquid crystal panels, automobiles, traffic lights, street lights, and image display units.
As electronic components become smaller and more sophisticated as described above, the amount of heat generation by a semiconductor device tends to be increasing. Unfortunately, exposure to a high-temperature environment for a long time will prevent an electronic component from functioning normally, and will also reduce its service life. Therefore, a bonding material for use in die bonding (a die-bonding material) usually includes a bonding material with high heat-dissipation performance to dissipate heat generated by a semiconductor device in an efficient way. Usually, depending on applications, a bonding material is required to have a function to allow heat generated by a semiconductor device to efficiency escape to a substrate and a housing, i.e., required to show high heat-dissipation performance.
As described above, high heat-dissipation performance is required for a bonding material used in electronic components, and accordingly, high-temperature lead solder containing a large amount of lead and gold-tin solder containing a large amount of gold have been traditionally and widely used. However, the high-temperature lead solder has a problem in that the solder contains lead which is considered as hazardous to humans. Therefore, a lead-free technology has been actively explored in recent years, and extensive researches have been conducted for switching to a lead-free solder. Gold-tin solder has a disadvantage in terms of cost because it contains gold which is expensive.
In view of the above circumstances, as a promising alternative material to replace high-temperature lead solder and gold-tin solder, an isotropic electrically conductive adhesive (hereinafter, may simply be referred to as “an electrically conductive adhesive”) has gathered attentions in recent years. An electrically conductive adhesive is a composite material of a metal particle having functions such as electrical conductivity (for example, silver, nickel, copper, aluminum, or gold) and an organic adhesive having an adhesive function (for example, an epoxy resin, a silicone resin, an acrylic resin, or an urethane resin). The types of the metal particle and organic adhesive used therein may vary. The electrically conductive adhesive is convenient because it is a liquid at room temperature, is free of lead, and is inexpensive. These make the electrically conductive adhesive a promising alternative material to replace high-temperature lead solder and gold-tin solder. Therefore, a significant market growth thereof is predicted.
As described above, an electrically conductive adhesive as an alternative material to replace solder is required to have electrical conductivity as well as high heat-dissipation performance. An organic adhesive used as a raw material of an electrically conductive adhesive basically has a lower thermal conductivity as compared with metal, and thus thermally conductive filler is blended to confer heat dissipation functionality on the electrically conductive adhesive. Minimizing the thermal resistance of an electrically conductive adhesive to effectively dissipating generated heat is the key for developing a technology with regard to electrically conductive adhesives.
As a conventional electrically conductive adhesive having improved thermal conductivity, for example, Patent Literature 1 discloses a composition having high thermal conductivity and electrical conductivity, the composition containing solid components of at least 5 to 80% by weight of a pitch graphitized carbon fiber filler having a mean fiber diameter of 0.1 to 30 μm, an aspect ratio of 2 to 100, a mean fiber length of 0.2 to 200 μm, and a true density of 2.0 to 2.5 g/cc; 15 to 90% by weight of a metal fine-particle filler having a mean particle diameter of 0.001 to 30 μm; and 5 to 50% by weight of a binder resin.
Further, Patent Literature 2 discloses an electrically conductive composition containing: an epoxy resin as a base resin; a phenol-based curing agent as a curing agent; a urethane-modified epoxy resin as a flexibility-imparting agent; and further a powder of, for example, gold, silver, copper, iron, aluminum, aluminum nitride, alumina, crystalline silica as a thermally conductive filler.
Furthermore, Patent Literature 3 describes an adhesive containing: a resin component; a fibrous filler having high thermal conductivity; and a spherical filler having high thermal conductivity including at least one selected from the group consisting of silver, gold, platinum, aluminum nitride, silicon oxide, aluminum oxide, and carbon black, in which the content of the fibrous filler having high thermal conductivity is 0.1 to 20 parts by volume, and the content of the spherical filler having high thermal conductivity is 10 to 200 parts by volume relative to 100 parts by volume of the resin component.
Moreover, Patent Literature 4 describes a non-solvent liquid silver paste composition containing a bisphenol-type epoxy resin, a liquid aromatic amine as a curing agent, and 85 to 95% by weight of a silver powder, in which the composition serves as an adhesive having high thermal conductivity used to bond a heat generating body such as a chip in a semiconductor device to a heat dissipating body such as a lead frame.
As electronic components are becoming smaller and more sophisticated as described above, taking an appropriate measure for conferring high heat-dissipation ability on an electrically conductive adhesive is an important issue in the industry, and thus an electrically conductive adhesive having a proper balance of both high thermal conductivity and electrical conductivity yet remains to be developed.
Accordingly, prior to the present application, the present applicant disclosed a thermally and electrically conductive adhesive composition including: (A) an electrically conductive filler; (B) an epoxy resin; and (C) a curing agent, in which the electrically conductive filler (A) is a submicron fine silver powder, and the content of the fine silver powder is 75 to 94% by mass relative to the total amount of the thermally and electrically conductive adhesive composition; and the content of the epoxy resin (B) is 5 to 20% by mass relative to the total amount of the thermally and electrically conductive adhesive composition; and the curing agent (C) is a compound represented by Formula (I), (II), or (III) (not shown herein), and the content of the compound is 0.4 to 2.4 molar equivalents in terms of equivalent of active hydrogen relative to 1 molar equivalent of epoxy groups in the epoxy resin (B); and during heat curing and before the electrically conductive filler (A) starts to sinter, the thermally and electrically conductive adhesive composition is in an uncured state or a half-cured state (Patent Document 5).